1. Field of the Invention
The present invention relates to an inkjet printing apparatus, which prints images by splitting a liquid material into droplets that are ejected to a print media, and a method of recovering a liquid material. In particular, the present invention relates to an inkjet printing apparatus, which continuously ejects droplets by means of the pressure of focused ultrasonic waves emitted from transducers, and a method of recovering a liquid material.
2. Related Art
Inkjet printing apparatuses, which eject liquid droplets toward print media to form printing dots, have such advantageous effects that less noise is produced as compared to other printing systems, and that it is not necessary to perform developing treatment and fixing treatment. Accordingly, inkjet printing apparatuses are widely used in the field of plain paper printing technology. Having such characteristic features that non-contact recording is possible, that printing can be performed with a minimum of materials consumed, and that it is possible to manufacture such apparatuses at a low cost, etc., the inkjet apparatuses are used beyond the conventional field of printing, i.e., printing images to paper media, and are applied to an industrial process field such as the coating of liquid electronic material, direct patterning, etc. In the fields of industrial process and industrial printing, the most important requirement is a high-speed throughput. In order to achieve this requirement, a high-speed droplet ejecting frequency, a highly dense positioning of nozzles, and high ejection reliability are required.
At present, various types of inkjet printing apparatuses have been proposed, of which representative examples are one that ejects droplets by means of the pressure of steam generated by heat from a heating element, and one that ejects droplets by means of pressure pulses caused by the displacement of a piezoelectric material.
In these types of inkjet printing apparatuses, droplets are ejected from a nozzle disposed at the end of a pressure chamber, which contains a printing liquid, by means of changes in pressure inside the pressure chamber. Such inkjet printing apparatuses are in actual use as so-called “on-demand type” inkjet printing apparatuses, which eject droplets in accordance with image printing information. However, in such on-demand type inkjet printing apparatuses utilizing wholly changes in pressure in the pressure chambers, there is a problem in that when droplets are ejected, the meniscus at the liquid surface, from which droplets are ejected, falls back, and a certain period of time is required for the meniscus to return to the original position by the refilling of the printing liquid from a printing liquid tank, resulting in that it is difficult to eject droplets at a high frequency. Furthermore, the adverse effect of vibrations remaining in the printing liquid pressure chamber makes it difficult to perform continuous ejections at a high speed. As a result, when droplets are intended to be continuously ejected at a high frequency, the ejections become unstable and certain phenomena may occur, such as no droplet being ejected, extra satellites (sub-droplets) being ejected, etc.
Although an on-demand type inkjet printing apparatus ejects droplets for the printing of an image upon receiving image printing information, there is another type of inkjet printing apparatus, i.e., so-called a “continuous type (continuous ejection type)” inkjet printing apparatus, which continuously ejects droplets but changes the flying directions of droplets upon receiving image printing information. This type of inkjet printing apparatus has a characteristic feature that high-speed printing is possible. A charge-control inkjet printing apparatus, which is a typical apparatus of the continuous type, includes a charged electrode which selectively charges droplets in front of the nozzle in accordance with image printing information, and a deflection electrode which deflects the flying direction of ejected droplets passing through it by an electric field. Although such a continuous type inkjet printing apparatus can continuously eject droplets at a high frequency, the structure thereof is complicated and a high voltage is required in order to operate it. Accordingly, it is difficult to densely position nozzles, and there is a limitation on the properties of printing liquid.
Another type of inkjet printing apparatus, i.e., an ultrasound inkjet printing apparatus, has also been proposed, which focuses ultrasound waves generated by a transducer in order to eject droplets from a surface of a printing liquid by means of the acoustic pressure of the ultrasound waves. Since such an inkjet printing apparatus is of a “nozzleless” type, which does not require nozzles each corresponding to individual dot, nor needs a partition wall for dividing printing liquid paths, it can effectively prevent the clogging and eliminate the step of recovering from the clogging, which have been an obstacle to the production of a “line head type” inkjet apparatus. Furthermore, since it is possible for this type of inkjet printing apparatuses to stably eject very minute droplets, they are suitable for improving resolution. Moreover, there is little limitation on printing liquid material used in this type of printing apparatus since the size of droplets is dependent on the wavelength of ultrasound waves. There is a problem, however, in that it is difficult for this type of printing apparatus to eject droplets at a high frequency since it is difficult for this type of printing apparatus to generate a power to pull back the meniscus formed at the liquid surface at a high speed after the ejection of droplets.
There is an ultrasound inkjet printing apparatus of continuous type, which ejects droplets by means of focused ultrasound beams, as disclosed in Japanese Patent Laid-Open Publication No. 248913/1997 (pages 2–5, FIG. 1). However, like the aforementioned continuous type charge-control inkjet printing apparatus, an inkjet printing apparatus of this kind becomes rather large due to the use of an electric field to control the courses of ejected droplets. In addition, in order to prevent the interference of electric field between adjacent droplet ejection portions, it is not possible for this type of inkjet printing apparatus to densely position droplet ejection portions.
There is an ultrasound inkjet printing apparatus of the on-demand type, which ejects droplets of printing liquid in multiple directions by combining a plurality of transducers generating ultrasound waves, as shown in U.S. Pat. No. 4,308,547. However, an inkjet printing apparatus of this kind has a problem in that the acoustic pressures of ultrasound beams focused on the liquid surface tend to vary depending on the directions of ejected droplets, thereby varying the sizes of droplets, resulting in that it is difficult for this type of inkjet printing apparatus to eject droplets in a stable manner.